Interesting VM feature?

(Please CC: me,)

Hi,

I've recently come across someone porting some code from windows that trys to implement an efficient memory pool using some VM tricks. I have no idea if linux has an equivalent to this feature (described below), and wouldn't know what to even search for to find out.

What this mempool wants to do is to be able to allocate a block of memory and tell the kernel which pages from it can be outright discarded, instead of swapped out when memory starts to get crowded.

Now, from looking at mel's docs, it looks to me like this would mean letting the application directly mark pages clean. But it would also mean finding any swapped out versions of this page and deallocating them, otherwise if the page is discarded and then used again, time would be wasted swapping in garbage.

I'm going to keep reading. If this is already implemented, or if the efficiency gains would be nil, somebody yell at me before I start crashing my kernel with attempts at doing this myself. :-) It seems so trivial after all...

Thanks,
Ted Kaminski

I think you might be able to get what you want with madvise() or perhaps by mmap()ing new clean pages on top of the pages you want to throw away.

I would not implement this unless you either know you have a problem with your mempool swap speed or you are bored. I doubt it is going to help a lot, and it will certainly simplify your code to leave it out. If you later find that you have performance problems you can look into this again.

Thanks,

Jim

You can call madvise(start, length, MADV_DONTNEED), or you can mmap() fresh empty pages into the region.

I have no idea if either of these methods is efficient enough to be useful. Also, I don't know whether mmap() would create multiple VMAs, or if it is clever enough to merge adjacent vmas of anonymous private mappings regardles of offset.

The ideal implementation would give the kernel the _option_ of discarding pages until they are next touched, so that they are discarded when there is memory pressure but retained if not, avoiding the unnecessary zero-fill and cache flush.

Unfortunately, though much discussed a long time ago, the kernel offers no service like this.

-- Jamie